This invention is in the field of split-cycle Stirling-cycle coolers which generally have a compression portion and expansion portion connected by a refrigerant conduit. The expansion portion has a "cold finger" in which expansion of the refrigerant occurs. This cold finger has an ambient end from which the refrigerant towards a cold end. Thus a temperature gradient will exist from the ambient to the cold end of the finger. Typical of such coolers are those shown in the U.S. Pat. Nos. 3,630,041, 3,862,546 and 3,877,239. An improvement in operation of such typical coolers will be realized if the temperature gradient along the cold finger is reduced. The present invention is able to reduce this gradient by providing enhancing motion of the expansion piston.